The present disclosure relates to optical coherence tomography systems and methods. More particularly, the present disclosure relates to optical coherence tomographic vibrography.
Optical coherence tomography (OCT) is an optical interferometric imaging technology that can produce depth-resolved images of sub-surface tissue structures. This is accomplished by taking a spatially coherent infrared light-source and splitting it between a reference beam and a sample probing beam. Light that is backscattered from structures within the sample are collected and interfered (combined) with the reference beam light in order to produce an interference pattern that, once processed, reveals the location of light-reflecting structures in the sample.
OCT measurements can be performed using various approaches, either in the time domain (TD-OCT) or in the frequency domain (e.g., Fourier transform approaches such as spectral domain [SD-OCT] or swept-source [SS-OCT]). The most recent of these to see significant advancements is SS-OCT, which is schematically illustrated in FIG. 1A. In SS-OCT, a wavelength-tunable laser 100 is used as the light source to probe the sample. By varying or “sweeping” the optical wavelength of light emanating from the laser, an interference pattern can be detected at many wavelengths and frequency analysis of the detected signal can be used to identify the z-position of a light-reflecting structure in a sample.
Referring to FIG. 1A, at each frequency, the laser fires through optical fiber 105 into a beam splitter 110 that splits the beam into reference 115 and sample 120 arms with a path length difference AD. The beam of the reference arm follows fiber to its exit and then is reflected by a mirror 125, while the beam of the sample arm follows fiber to its exit and then contacts an area on the sample. Structures within the sample area may then reflect some of the beam back to the fiber. Both beams are added together at the beam splitter, photodetected at photodetector 130, amplified by amplifier 135, and recorded in a computer (not shown). As optical frequency, v, is swept in time, oscillations are generated in the measured interferogram 140 with frequencies that are proportional to the path length differences in the two arms of the interferometer. A hypothetical depiction of such an interferogram is shown in the frequency domain (amplitude over time, 140), which is transformed into an A-line 145 (amplitude over distance, bottom right) using discrete Fourier analysis software running on the computer.
The interference pattern as measured by a photodiode contains oscillations in time whose frequencies are proportional to the depths of the reflectors in the sample. By occasional sampling of the interference patterns produced by an arbitrary wavelength sweep profile such that the sampling occurs at evenly spaced optical frequency intervals, or by sweeping the laser linearly in optical frequency and regularly sampling the interference pattern, etc., a reflectivity depth-profile of the sample along the beam path (called an “A-line” or “A-scan”) can be obtained by taking the magnitude of the discrete-Fourier-transform (DFT) of the sampled interferograms. 2D (x by z, where z is defined along the axial direction of the beam) brightness mode images (called “B-mode” or “B-scan”) can be constructed by scanning the beam across a field-of-view in x and stitching together adjacent A-Lines into an intensity map. Similarly, 3D B-mode volume renders of structures can be constructed from a stacked set of 2D B-mode images collected at various y positions. In medical diagnostics, B-mode images provide anatomical information, i.e. the ability to discern normal structures from pathological ones.
In Spectral Domain OCT (SD-OCT), shown in FIG. 1B, a beamsplitter 110 splits light from a broadband source 200 between a reference arm 115 and a sample arm 120 and the light reflected from the two arms is interfered at another beamsplitter (which, in some embodiments is just the first beamsplitter used again). The interfered light is dispersed using a dispersive optic 150 such as a dispersion grating and the spectrum of the signal is recorded using a photodetector array (line camera) 155. The spectrum 142 is the Fourier transform of the axial scan line 145 (A-line) giving the reflectivity of the tissue as a function of depth.
OCT can also be used to perform functional measurements in tissue. The magnitude of the DFT of the interferograms contains structural information about sub-surface reflectors, and the phase of the DFT contains dynamic information. Repeatedly acquired A-lines at the same x,y position of moving objects will contain phase differences that reflect the structures' motion in z. Phase-sensitive OCT (PS-OCT) systems derive additional image contrast from this phase information and can quantify dynamics, and are often referred to as Doppler Optical Coherence Tomography systems.
OCT has been applied to imaging the human tympanic membrane and middle ear. It has been shown that anatomical structures within the middle ear can be imaged using OCT; that tympanic membrane can be imaged in patients using OCT; and that PS-OCT can be used to perform functional imaging in the human middle ear by measuring the vibration of middle ear structures in response to sound. To date, the basic approach that has been taken to extracting magnitude-of-vibration information in non-real-time, benchtop PS-OCT relies on an acoustic stimulus that is applied to the ear; the acoustic frequency phase variations are then collected over many consecutive complete acoustic cycles and analyzed using Fourier analysis.
In order for meaningful information to be extracted from the changes in phase of the sampled interferograms, PS-OCT requires a high degree of phase stability. As such, performing PS-OCT in the time domain is difficult to implement and is incompatible with real-time imaging. PS-OCT is very compatible with SD-OCT as its lack of moving-parts and tuning mechanisms inherently provide very high wavelength repeatability, however “PS-SD-OCT” has been limited in scanning range due to complex-conjugate ambiguity and sensitivity-roll-off, making it less-attractive for use in imaging applications requiring more than a few millimeters of scanning range such as middle ear imaging. An important requirement for interferometric phase stability in SD-OCT and SS-OCT is wavelength-repeatability.
The recent availability of tunable lasers with long-coherence lengths has made SS-OCT a preferred approach for long-range imaging in the human middle ear. However, conventional tunable lasers used for SS-OCT (e.g., external cavity lasers tuned with, for example, polygon mirrors) suffer from a number of limitations. They exhibit phase instability owing to non-repeatability in mechanical tuning mechanisms. They suffer from electronic timing jitter due to the difficulty in adequately synchronizing laser sweeps with mechanical mirror positions and so require either a phase-reference reflector to be placed within the image, or for synchronization pulses to be generated by optical means. There remains a need to develop systems and methods that would allow for phase-sensitive OCT.